1. Field of the Invention
The invention generally relates to improved fixation of a prosthetic component to bone and, in particular, relates to an apparatus for and method of pressure injection of low viscosity bone cement into bone surfaces to improve the bone/cement interface.
2. Description of the Prior Art
Total arthroplasty has become widely accepted as a useful measure in the treatment of severe arthritis. It facilitates the correction of deformity, the reestablishment of stability and, most important, the relief of pain. Unfortunately, the procedure is associated with a number of complications including loosening of the arthroplasty components. A number of factors contribute to loosening including failure to correct a deformity, an overweight or overactive patient, an osteoarthritic joint in comparison to a rheumatoidal joint and the use of a constrained prosthesis. The consequences of loosening include return of pain and deformity, the need for further reconstructive surgery with attendant technical difficulties and increased risk of sepsis and the possible risk of subsequent re-loosening. Failure of the fixation may originate at the interface between the prosthesis and cement, at the interface between the cement and bone, or both.
Loosening of the joint arthroplasty components occurs in a number of ways. Resorption of the bone commonly occurs, particularly, around the stem of a constrained prosthesis and is the result of high loads placed on the cement/bone interface leading to micromovement, resorption and gross loosening. Separation of the cement from the bone is usually due to a weak bone/cement interface and results in gross displacement of the implant. A radiolucent line is commonly seen between the cement and bone and has the following characteristics. Microscopic examination of the interface between the cement and bone indicates that the radiolucent line corresponds to a fibrous membrane which has developed at the interface. Mechanically, the fibrous membrane presents a plane of weakness between the cement and bone and a potential site for loosening. Micromovement occurs at those interfaces where a fibrous membrane is interposed. The radiolucent line, therefore, represents a site of impaired mechanical integrity.
Failure of the cement/bone interface may be due to a mechanically incompetent interface from the moment of implantation. The doughy cement presently employed and having a viscosity at 68.degree. to 70.degree. F. of greater than 4000 poise at the 5th or 6th minute after mixing is applied to cancellous surfaces by hand and penetrates trabeculae in the bone only to a limited degree, if at all. More practically it may conform to surface irregularities, but does not penetrate the bone. Poor fixation of this type leads to micromovement which in turn leads to bone resorption which results in the development of a fibrous membrane. Acrylic cements as currently used for the fixation of joint arthroplasty components to bone has inherent deficiencies in terms of establishing secure and enduring interfaces between the prosthesis and the cement and between the cement and the bone.
Guns which are presently available for injecting cement generate a low pressure and are prone to blockage by the thickening cement. Therefore, the low pressure guns of the prior art are ineffectual in inducing cement to penetrate bone.
In addition, these guns are non-modular and are not suitable for a variety of tasks other than introducing cement into the upper end of the femur without pressurization.